There are already known various constructions of pumped optical waveguide lasers, among them such utilizing fully or partially reflective coatings on the respective end faces delimiting the laser resonance cavities in solid material optical waveguides, and such using external reflectors to delimit the laser cavities. Even though these solutions have achieved quite a degree of success and/or acceptance in the laser manufacturing and utilization fields, they still suffer of certain drawbacks, one of which is that, because reflecting elements of the above type are reflective over a relatively wide frequency bandwidth, lasing often occurs in more than one mode. This is true even if the optical waveguide, such as an optical fiber, is of the so-called single mode type in which all transverse modes but one are below the cutoff. As a matter of fact, experience has shown that even such a single-mode optical fiber laser typically lases in a great number of longitudinal modes. This, of course, significantly degrades the properties of the laser radiation issued by the optical fiber laser due to mode beating and other optical effects, thus considerably reducing the utility of the issued laser beam. This multiple longitudinal mode operation diminishes if not defeats utility of such lasers for many applications in the telecommunications, sensor and spectroscopic evaluation fields.
Narrow-linewidth single-frequency semiconductor laser diodes are currently the source of choice in low to moderate power coherent systems. In laser diodes, single-frequency narrow-linewidth operation is typically achieved through a distributed feedback or a distributed Bragg reflector structure. Moreover, miniature, diode-pumped, single-frequency, narrow-linewidth solid state lasers are currently being manufactured in a variety of configurations. Both of these laser types, however, suffer of certain drawbacks, an important one of which is that, when they are to be used in optical fiber systems, they require integration into such systems that is alignment insensitive, and hence expensive. Therefore, for these applications, it would be preferable to use optical fiber lasers. However, experience has shown that it is not at all easy to achieve the desired single longitudinal mode operation in linear optical fiber lasers, especially when they are to have substantial lengths. This is so because, even when the reflectors axially delimiting the laser cavity are of the relatively narrow passband type, still the number of longitudinal modes falling within this passband and thus in competition for lasing is substantial when the length of the laser cavity is in excess of a few millimeters or centimeters. Because of this, it was heretofore very difficult if not impossible to provide a linear solid material optical waveguide, especially an optical fiber, laser that would lase exclusively in a single longitudinal mode.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide pumped optical fiber laser arrangements which do not possess the disadvantages of the known arrangements of this kind.
Still another object of the present invention is so to develop the arrangements of the type here under consideration as to achieve lasing in only a single longitudinal mode.
It is yet another object of the present invention to devise arrangements of the above type which are able to achieve such single mode operation at various cavity lengths and gains of the optically excitable material of the optical waveguide.
A concomitant object of the present invention is to design the optical waveguide lasers of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use and integrate into optical fiber systems, and yet reliable in operation.